In the production of plywood, particle board, pressed board, and the like as described in U.S. Pat. No. 3,860,381 it is standard practice to use a platen press having a pair of platens with confronting parallel pressing surfaces. The mat or sandwich to be pressed into the desired workpiece is laid on the upper pressing surface of the lower platen and the two normally heated platens are brought together to compress this mat or sandwich to the desired extent.
Normally the spacing between the two surfaces of the press platens is strictly controlled by means of spacer bars that normally are provided at the edges of the platens. These bars, which are fixed to one of the platens, engage the other platen when the press is closed so as to maintain a predetermined distance between them. This distance in turn corresponds to the desired board thickness plus a relatively short distance equal to the so-called finish increment. This finish increment is normally subsequently removed by surface-treating the board. Under any circumstance this finish increment must be very exactly established, normally to an accuracy of tenths of millimeters.
Normally the spacer bars are provided for establishing this board thickness plus the finish increment. Thus it is necessary that each press is provided with an entire set of very closely dimensioned spacer bars, and that each time the pressing operation is changed the spacer bars are switched for other spacer bars. The amount of spacer bars necessary is equal to the number of different board thicknesses times the number of different finish increments times the number of spacer bars per platen. Thus it is necessary to provide an enormous and relatively expensive stock of such spacer bars.
It has been suggested in German patent publications Nos. 1,216,532 and 2,027,806 to eliminate these different spacer bars and provide a wedge-type adjustable spacer bar. Such a spacer bar is formed by a pair of wedges which are fitted together to form an adjustment bar having parallel outer surfaces, but with the wedges engaging each other at a surface that is inclined to these parallel outer surfaces. Means such as a threaded spindle is provided for displacing one of these wedges relative to the other to vary the overall thickness of the spacer bar. Since, however, the average board thickness varies between 5 and 30 mm, whereas the average finish increment must be adjusted to a tenth of a millimeter, it is relatively difficult with such a system to set the wedge-type spacer bar at the exact desired dimension. Due to the relatively steep angle necessary on the inclined plane between them a considerable vector of force is effective during pressing against the adjustment mechanism, which force frequently maladjusts it and caused the workpieces to be made substantially thinner than is desired. As a result this type of system has found little wide-range use and, even when used, requires continuous monitoring of board thickness to ensure that it has not maladjusted itself.